1. Field of the Invention
The present invention is directed to light emissive materials incorporating quinolinolato metal complexes used as emissive materials in electroluminescent (EL) devices, and in particular organic light emitting devices (OLEDs). These devices have utility, for example, in flat panel displays.
2. Description of the Related Art
OLEDs are typically comprised of at least a layer of emissive material sandwiched between an anode, typically comprised of a transparent conductor such as indium-tin oxide, and a cathode, typically a low work-function metal, such as magnesium, calcium, aluminum, or the alloys thereof. When a bias is applied across the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the emissive layer. The holes and the electrons form excitons in the emissive layer which emit light. Hole transport layers and electron transport layers may also be added adjacent the respective electrodes to facilitate charge transfer. Depending upon whether hole transport or electron transport is favored, the light emissive layer may be located closer to the anode or the cathode. In some instances, the emissive layer is located within the hole transport or electron transport layer. Known arrangements of electrodes, hole transport layers, electron transport layers and emissive layers in multilayer structures are disclosed for example in B. R. Hsieh, Ed., xe2x80x9cOrganic Light Emitting Materials and Devices,xe2x80x9d Macromolecular Symposia, 125, 1-48 (1997), which is incorporated herein by reference.
Tris(8-hydroxyquinoline)aluminum (AlQ3) complex is a widely studied emissive aluminum complex having the following structure: 
AlQ3 which has a characteristic green emission with a wavelength of about 535 nm, may be doped with guest emitter compounds to prepare an emissive system having an emission spectrum close to that of the guest. The emitter is energized by direct excitation or by transfer from the AlQ3 host. Examples of these systems are described in C. W. Tang, et al., J. Appl. Phys., 65, 3610 (1989), which is also incorporated herein by reference.
Conventional doping causes certain problems. For example, if the guest compounds do not disperse properly in the matrix, xe2x80x9caggregationxe2x80x9d occurs, local areas of high concentration of the guest compound which in turn leads to xe2x80x9cquenching,xe2x80x9d a phenomenon in which the guest compound absorbs energy but fails to emit at its characteristic wavelength or desired intensity.
AlQ3 has also become the prototype for a class of photoemitting materials in which quinolinolato metal complexes are bonded to organic groups. Examples of this class of materials are disclosed in U.S. Pat. Nos. 5,466,392 and 5,294,869, which are also incorporated herein by reference. Some of these materials show promise for use as emissive layers in OLEDs, exhibiting properties such as good electron transport, photoemission, high thermal stability, solubility and ease of sublimation. However, these photoemitting materials do not luminesce at wavelengths characteristic of the organic groups bonded to the metal complexes. Instead the organic groups merely modify or shift the emission of the metal complex portion of the material. While some of the organic modifying groups disclosed in the aforesaid U.S. Pat. Nos. 5,466,392, and 5,294,869 may have weak emission spectra, most of them are not light emitting at all.
The inventors herein have discovered materials based on the bonding of modified quinolinolate ligands to light emitting arylates, which exhibit photoemissive and charge transport properties, and which can reduce or eliminate the necessity for doping to improve emission efficiency or color in an organic light emitting device. In preferred embodiments, these materials exhibit emission spectra close to the characteristic emission spectra of the contained light-emitting arylate. In these instances, the metal complex serves to activate the emission of the light-emitting arylate, without contributing substantially to emission.
The invention is a light emissive material suitable for use in an OLED with a structure (L2M)nxe2x80x94X wherein L is a 2-alkyl-8-quinolinolate or 2-phenyl-8-quinolinolate ligand, M is a trivalent metal atom complexed with the ligand to form a conjugated metal complex, and n is an integer between 1 and 12. When n is equal to 1 or 2, X comprises a monovalent or divalent arylate emitter, respectively, that contains at least one arylamine group, or which is conjugation-isolated from the metal complex. When 3xe2x89xa6nxe2x89xa612, X is an n-valent light emitting arylate group.
In embodiments, when n is equal to 1 or 2, X is a light emitting group having an emission in the range of about 500 nm to about 750 nm. Emission at this wavelength is necessary to obtain energy transfer from the metal complex. Provided the emitter""s characteristic wavelength of emission is high enough (between about 500 nm and about 750 nm), the characteristic emission of the combined complex-plus-emitter will be at the characteristic emission wavelength of the emitter. In other embodiments, the complexes according to the invention comprise an amine or alkane functionality isolating the conjugation of the arylate emitter from the conjugation of the metal complex.
The invention in a further aspect is embodied as a multivalent arylate emitter core bonding to 3 to 12 metal quinolinolate complexes according to the following structure: 
where xe2x80x9cMxe2x80x9d is a trivalent metal and xe2x80x9ccorexe2x80x9d is a multivalent moiety such as: 
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings.